The majority of power plants that have been installed during the last several decades have been the type which employ a natural gas- or oil-fired steam boiler. Conventional coal-fired boiler plants have become more or less obsolete during this period since both natural gas and oil fuels have been more convenient to use than coal. However, the cost of natural gas and oil has steadily increased in the last several years. Additionally, there is a serious shortage of these fuels in most countries throughout the world. Coal, on the other hand, is still an abundantly available fuel and its cost is moderately low. Consequently, coal-fired steam boiler plants that are in operation today constitute an important part of industry's base line power generation capacity.
In the operation of a conventional coal-fired, suspension type steam boiler, pulverized coal is suspended in an air stream and is fed through conduits to a multiplicity of coal burners in the steam boiler. Upon emerging from the burners the coal is usually mixed with additional air and is ignited. Current practice is to ignite the pulverized coal by means of natural gas or oil fired igniters.
It is contemplated that in future years coal-fired steam boiler plants will be used in conjunction with nuclear plants for generating electricity during times of peak demand. In these installations, the coal-fired steam boilers can be operated under so called "turn-down" conditions, i.e. operation at less than about 70% of design point power. Under these conditions, the pulverized coal that is fed to the coal burners tends to flow intermittently from the burner nozzle causing the coal-air flame to be extinguished between slugs of coal. Combustion instabilities can be created if the flame is not immediately reignited. Moreover, any coal that is not ignited may be swept into hidden recesses in the boiler where the coal can create an explosion hazard. It may therefore be necessary to continuously operate the natural gas or oil fired igniters so as to reignite and stabilize the flame during periods when the coal-air flame might be extinguished.
Depending on the particular size of the steam boiler, there may be anywhere from twelve to forty-eight coal burners employed. Each coal burner may typically range in size from around 20 megawatts per burner to as high as 50 megawatts. Since the natural gas- or oil-fired igniters commonly have thermal ratings which may range from between about 2% and 20% of the thermal rating of the main burners, it will be seen that the operation of these igniters on a continuous basis can be very expensive. Moreover, they will become even more expensive to operate as the cost of liquid and gaseous hydrocarbon fuels escalates in future years.